Low-frequency loudspeaker boxes, so-called subwoofers, frequently contain an integrated bass management which includes a frequency separation between the subwoofer and other loudspeakers or loudspeaker boxes connected to the subwoofer. This can be, for example, so-called monitor boxes in the studio area which are also simply designated as monitors. Unless expressly specified otherwise, the term “loudspeaker” in this description also covers loudspeaker boxes. It is known to carry out a phase matching for the loudspeaker boxes connected to the subwoofer in order to match their phase to the phase of the subwoofer. Otherwise, i.e. if the phases are not matched, frequency-dependent delays are formed between the sound reproduced by the loudspeaker boxes and the sound reproduced by the subwoofer. In a transition frequency range in which the subwoofer and the other loudspeaker boxes deliver sound signals in comparable amplitudes, the superposition of these sound signals—it they do not match one another in their phase position—can have the result that the sound signals delivered by the subwoofer and the sound signals delivered by the other loudspeaker boxes exaggerate or partially cancel out each other in a frequency-dependent manner which has a perturbing effect in the hearing impression. The matching can be carried out by allpass filtering or delay in the range of the frequency separation between the subwoofer and the other connected loudspeaker boxes.
In addition, it is known to carry out a phase linearization of the natural phase nonlinearity in the range of the frequency separation in multiway boxes. This is achieved, for example, in two-way boxes (low-frequency/high-frequency) or three-way boxes (low/medium/high-frequency), in analogue systems by filters with an allpass component or in digitally controlled systems by FIR filters.
In this case, a low-noise implementation of phase linearization in loudspeaker boxes frequently requires a digital, i.e. DSP-based signal processing with FIR filters. A subsequent phase linearization of existing loudspeakers is only possible by adding a proprietary external electronic circuit which behaves inversely to the phase behaviour of the respective loudspeaker.
FIG. 1 shows a known analogue loudspeaker 100, in particular a two-way box with analogue signal processing. The audio data received at the input 105 are split in an audio crossover comprising a high-pass filter 110 and a low-pass filter 140 into a low-frequency and a high-frequency component which are amplified via dedicated amplifiers 120, 150 and output to corresponding loudspeakers 130, 160 suitable for the respective frequency range. In this case, the analogue high-pass filter 110 normally has a phase response in its frequency response 110a in which for frequencies below the corner frequency the phase is rotated by +90° (for first-order filter) whereas for frequencies above the corner frequency the phase is not rotated (i.e. 0°). On the other hand the analogue low-pass filter 140 normally has a phase response in its frequency response 140a in which for frequencies below the corner frequency the phase is not rotated whereas for frequencies above the corner frequency the phase is rotated by −90° (for first-order filter). In the acoustic sum the loudspeaker box with its frequency response 100a delivers a constant amplitude in its envisaged working range, i.e. above a lower limiting frequency. As a result of the above-mentioned phase linearization, it can also be achieved that the phase is constant above the lower corner frequency so that the loudspeaker box is designated as “phase-linear” above this frequency. At the lower limiting frequency and below, the analogue loudspeaker box 100 exhibits a similar behaviour to a high-pass filter because the phase is changed by up to +90°.
FIG. 2 shows a known digital loudspeaker 200, in particular a digitally controlled two-way box. The audio data received at the input 205 are again split into a low-frequency and a higher-frequency component in an audio crossover comprising a digital high-pass filter 210 and a digital low-pass filter 240, which components are each amplified via dedicated amplifiers 230, 260 and output to corresponding loudspeakers 230, 260 suitable for the respective frequency range. In this case, both the digital high-pass filter 210 with its frequency response 210a and also the digital low-pass filter 240 with its frequency response 240a are designed to be phase-linear. This means that both frequency responses 210a and 240a have a phase of 0° in the respective processed frequency range, wherein the phase-linear filtering however produces a delay DM1. The delay DM1 is not shown in the phase response in this analysis but is treated separately. The digital loudspeaker box 200 also has a natural lower limiting frequency. In the acoustic sum the digital loudspeaker box 200 with its frequency response 200a has an amplitude response like a high-frequency filter but the phase in the processed frequency range is 0° and the output is overall delayed by the delay DM1.
This difference in the behaviour of analogue and digital loudspeaker boxes close to the respective lower limiting frequencies results in problems when these are to be operated together with a subwoofer because the bass management for the subwoofer must intervene precisely in this frequency range.
In the priority-substantiating German patent application, the German Patent and Trademark Office has searched the following documents: DE 39 28 122 A1, DE 42 24 404 A1 and US 2004/0 258 256 A1.